This is a Scam!!  No one on the line, (robocall) when I initially answered.  They called back and a guy came on the line stating:... "Yeah, thought this was your fax number"..
Do file a complaint on the 'do-not-call' list, but more importantly file an online complaint directly with the FCC:
http://www.fcc.gov/complaints

Phone rings number appears on caller id says unknown but when you answer it hangs up.

calls several times a day. leaves no messages.

actually had to do star 69...don't have caller id...phone's been ringing and not leaving messages for two days...

calls several times a day, all day

This number has appeared on my caller ID a lot recently, though I have never been home to answer the phone.

Some telemarketing companies use autodialers to call you, often multiple times a day. When you answer the phone, all you get is a hang up. The autodialer then logs when you answered the phone, versus when you didn't answer the phone. This information allows an actual human telemarketer to call you when you are actually home.

For instance, if you see the number appearing on your caller ID all day, but you aren't home to actually answer the phone, the human telemarketers aren't prompted to call you during those times. If, however, you answer the phone, it gets logged in the system that this is a good time for a human telemarketer to call you in the future because you are likely home.

Not sure if this is the case here, but it sounds like it...

They are a fire fighters fund raising organization.

Just spoke with a male claiming to be soliciting donations for children that are in the burn unit. Claimed to be on behalf of the local Fire Dept. He was stumbling on his words alot, wasn't sure I believed him. Told him to send the info to me (he already had my address) I will research it when I receive it)He asked how I would be paying & He wanted me to ensure I would follow up & actually send a check.

They told me local fire dept. Questioned them and they humg up.

I received the same thing

This number is the state police trying to get you to give money for the state troopers

1. "Unknown Caller"
2. No audio at all.
3. Immediate hangup when I ask who it is.
4. Fairly frequent.  Several times per day.

The phone rings and the number comes up on caller ID but it says unknown name.  When it goes to the answering machine, there is silence then it hangs up.